While there is a list of freshwater fish that can live in salt water and vice versa, in general, saltwater fish will die in fresh water. This is because of osmosis and the way that fish absorb water through their skin. Saltwater fish die in fresh water due to overhydration, and freshwater fish die in salt water due to dehydration.
The process of osmosis
It is important to understand osmosis to fully comprehend why fish can live only in fresh or salt water depending on the species. Osmosis describes the way that fluids move across a semipermeable membrane. These membranes, such as the membranes surrounding a fish's cells, can allow the fluid molecules to pass through it but do not allow larger molecules, like salts, to pass through.
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The water moves across the membrane from the side with the lower concentration to the side with the higher concentration of salts. If the concentration of salts is the same on both sides of the membrane, the fluid does not need to move.
Osmoregulation in fish
So, how does osmosis apply to fish? The concentration of salts in the water determines whether the water flows in or out of the fish's cells. Salt water is considered hypertonic to fish. This means that the concentration is greater in the salt water than in the fish's cells, and water flows out of the cells. The reverse is true in fresh water. Fresh water is hypotonic, and water will flow into the cells.
Fish are adapted to handle this flow of water. Saltwater fish must frequently drink water to rehydrate their cells and accommodate for the moisture that is lost. Their gills and kidneys filter excess salt from the bloodstream, and the fish urinate out the waste. Because freshwater fish aren't adapted to drink water or process that much salt, they can quickly become dehydrated and die when placed in salt water.
Freshwater fish, on the other hand, do not need to drink water, as water is constantly flowing into their cells. These species get rid of excess water through frequent urination. A saltwater fish placed in fresh water is unable to remove the excess water fast enough. The fish can quickly become bloated and die when placed in fresh water.
List of freshwater fish that can live in salt water
There is a short list of freshwater fish that can live in salt water for a period of time. Catadromous fish, such as the European eel, live in fresh water but travel to salt water to spawn. Anadromous fish, such as the striped bass and salmon, live most of their life in salt water. However, they are born in fresh water and return to fresh water to lay eggs.
Freshwater fish that live in brackish water can also survive salt water conditions. Some examples include catfish, guppies, mollies, and puffers.
Fish care in an aquarium
Improper water salinity, such as fresh water for a saltwater fish and vice versa, is just one reason your fish may be dying. There are other water quality considerations to take into account. An abrupt change in water chemistry may occur after cleaning a tank and can kill your fish. In addition, too much nitrate and ammonia can be toxic to fish. Check water quality and pH regularly and make sure to remove chlorine from tap water when you are changing out the water in the tank.
Be sure to also monitor the temperature in the tank. Some species can only survive at certain temperatures, so make sure the tank's water heater is functioning properly. Ensure that the water temperature is in the ideal range for your fish before you return them to the tank after cleaning.